The best way is to have all the income shown on the tax returns!
The sellers who don’t report all the quarters that “fall on the floor” show a lower gross income. A lower gross income means a lower net income. And that is what a buyer is looking to buy - the net income.
Of course, you can estimate the gross income based on the cost of the utilities pretty closely, but not all buyers are that savvy and are willing to do that. Instead, the store gets sold at half of what it could have sold for.